When using gas turbines for vehicle propulsion and for other purposes with widely varying power requirement, it has been found difficult to obtain a reasonable degree of efficiency over a sufficiently large load area. A considerable improvement has been reached, however, by arranging an expansion turbine with adjustable inlet guide vanes on the compressor shaft in front of the compressor, and in this way the gas turbine can attain good efficiency down to one fourth of the full load, at the same time as the adjustable inlet guide vanes for the expansion turbine constitute a convenient control member for the gas turbine. At full load and accordingly full opening of the inlet guide vanes, however, the conditions of flow in the expansion turbine will be less suitable if the expansion turbine continues to operate at the same speed as the compressor, since the expansion turbine in this case tends to work as a compressor, for which the turbine blade profiles are very inconvenient. For this reason the expansion turbine is generally connected to the compressor by way of an axial clutch which is released when the guide vanes are fully opened, whereafter the expansion turbine is allowed to idle at full speed. Inspite of such arrangement however, the expansion turbine produces quite substantial throttling losses, with consequent deterioration of the efficiency of the whole unit.